1. Field of the Invention
The present invention concerns a time-varying magnetic fields generator for a magnetic resonance apparatus, and a magnetic resonance apparatus with such a generator.
2. Description of the Prior Art
Magnetic resonance is a known technology to acquire images of the inside of a body of an examination subject. In a magnetic resonance device, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static homogenous basic magnetic field that is generated by a basic field magnet. The magnetic resonance device also has a radio-frequency system that radiates radio-frequency energy into the examination subject to excite magnetic resonance signals, and that acquires the generated magnetic resonance signals, on the basis of which magnetic resonance images are produced.
A magnetic resonance apparatus is known from German OS 44 14 371 in which a radio-frequency shield is disposed between a radio-frequency antenna and a gradient coil system, this radio-frequency shield being fashioned to be permeable for the electromagnetic fields generated in the low-frequency range by the gradient coil system and impermeable for the fields generated in the radio-frequency range by the radio-frequency antenna. The radio-frequency shield has a first electrically-conductive layer arrangement and (additionally) a second electrically-conductive layer arrangement disposed opposite the first, these electrically-conductive layer arrangements being separated from one another by a dielectric. Each layer arrangement has adjacent conductor runs that are separated from one another by electrically insulated slots; the slots in the first layer arrangement being displaced offset relative to those in the second layer arrangement. In at least one layer the adjacent conductor runs are connected with one another via specially arranged bridges (including, for example, capacitors) that conduct substantially only high-frequency currents.
The radio-frequency antenna of the magnetic resonance apparatus is, for example, fashioned in a form known as a birdcage antenna. A birdcage antenna, for generating a homogenous radio-frequency field within a volume enclosed by it, normally is fashioned such that longitudinal conductors that are connected with one another by ferrules (rings) disposed parallel to one another and equally separated to define a cylinder surface. Tuning ensues in the high-pass and low-pass filter ranges, by capacities introduced for each of the longitudinal conductor in the ferrules between adjacent conductors such that a homogenous radio-frequency field results at resonance. Embodiments of such a birdcage antenna are, for example, disclosed in U.S. Pat. No. 4,680,548.
A magnetic resonance apparatus is also known from German OS 101 14 319 in which an antenna conductor of a radio-frequency antenna of the magnetic resonance apparatus is designed to form a hollow space, and the hollow space is provided to accommodate a shim device of the magnetic resonance apparatus.
A magnetic resonance apparatus with a gradient coil system is known from German OS 101 56 770 in which an electrically-conductive structure is arranged and fashioned such that at least within in imaging volume of the magnetic resonance apparatus, a 3D field pattern of a magnetic field produced by a gradient field via induction effects is geometrically similar to the 3D field pattern of the gradient field. In an embodiment at least one part of the structure is barrel-stored as a component of a basic field magnet. Among other things, this gradient coil system can be advantageously fashioned without gradient shielding coils, because the unwanted consequences of the switched gradient fields can be nearly completely controlled by a pre-distortion due to the geometric similarity of the magnetic field caused by the structure, such that no diminution of the gradient fields occurs due to the gradient shielding coils.